For parents who lost their children in the deadly shooting at Sandy Hook Elementary School in Newtown, Conn., on Dec. 14, 2012, the news of an armed man entering an Atlanta-area elementary school on Tuesday served as a reminder of the long struggle ahead for enacting gun reforms.

In the eight months since they last saw their children, Barden and Márquez-Greene, along with other parents of Newtown victims, have been campaigning for gun law reforms — including mandatory background checks and magazine size restrictions, access to mental health care and promoting gun safety.

Gun legislation passed in two states, Connecticut and Colorado, but failed at the federal level.

Barden said he felt frustrated, but not discouraged.

Nelba Márquez-Greene lost her 6-year-old daughter, Ana, in the deadly shooting at Sandy Hook Elementary School in Newtown, Conn.Ann Curry

"But it was also round one. And I feel like we are in mile one of a marathon. And, as Nelba said, change is coming," he said.

"I'm in this for the long haul," Márquez-Greene added, saying their goals are not only related to gun responsibility but also to improving mental health services.

"My son Daniel was known by his teachers as an unusually empathetic little boy who would notice somebody sitting alone and ask to go comfort that person," Barden said.

"And the sad parallel is that the individual that killed our children was also that little boy that sat alone. And if someone had gone and sat with him and showed him that they cared, maybe this could be very different," he added.

No one was injured when police say 20-year-old suspect Michael Brandon Hill walked into the front office of Ronald E. McNair Discovering Learning Academy in the Atlanta area with an automatic weapon.

"Our prayers were answered yesterday when all of those families were able to be reunited with their children," Márquez-Greene said.

"I was thinking about the back-to-school checklists that come around this time for so many parents and caregivers. And I think after yesterday, I was thinking backpack, check. Pencils, check. Coffin? I don't want anybody else to have that thought. And that's why we do what we do."